The real key would be if there was a significant drop in the amount of silicon wasted in the 12" vs 8" wafers as it also stands to reason that the 12" wafers are going to be more expensive than the 8" wafers (a 12" wafer is 225% of the area of an 8" wafer.)
There are 22 FF potential site on an 8 in wafer and 56 potential site on the 12 in wafer. The major (90% +/-) cost of the finished wafer is in the processing, not the starting wafer. So there is 245 % gain in potential site between the 12 in and 8 in wafer. Let us assume that the cost of 12 in finish wafer is 150% of the 8 in wafer (I am being generous here, It can be as low as 125%, depends on the wafer fab). Then the FF sensor cost from the 12 in wafer will be 60% of the 8 in wafer.